It has been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond 20 MHz will be based on the aggregation of a plurality of component carriers at different frequencies. Such carrier aggregation can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at 800 MHz, 2 GHz, and 3.5 GHz). Whilst a legacy mobile device may only be able to communicate using a single, backward compatible, component carrier, a more advanced multi-carrier capable terminal would be able to simultaneously use the multiple component carriers.
Carrier aggregation can be particularly beneficial in a heterogeneous network (HetNet), even when the system bandwidth is contiguous, and does not exceed 20 MHz because multiple carriers enable interference management between different power class cells as well as open access and closed subscriber group (CSG) cells. Long-term resource partitioning can be carried out by exclusively dedicating carriers to a certain power class of cell (Macro/Pico/CSG).
Further, the need for interference management between different cells operating on component carriers of the same frequency in co-incident or overlapping geographic areas has led to the development of extension carriers (which are not backwards compatible with legacy devices). Extension carriers may be used as a tool for carrier aggregation based HetNet operation and improved spectral efficiency. A multi-carrier capable base station is able to operate at least one of its carriers as an extension carrier, on which a control channel (e.g. a channel carrying resource scheduling information such as the Physical Downlink Control Channel (PDCCH)), a Common reference Signal (CRS) (sometimes referred to as a Cell-specific Reference Signal), and other information cannot be transmitted. More specifically, an extension carrier may not be used for transmission of any of the following:                a Physical Downlink Control Channel (PDCCH);        a Physical Hybrid ARQ Indicator Channel (PHICH);        a Physical Control Format Indicator Channel (PCFICH);        a Physical Broadcast Channel (PBCH);        a Primary Synchronization Signal (PSS);        a Secondary Synchronization Signal (SSS); or        a Common Reference Signal/Cell-specific Reference Signal (CRS).        
An extension carrier therefore comprises a carrier that cannot be operated as a single carrier (stand-alone) carrier, but must be a part of a component carrier set where at least one of the carriers in the set is a stand-alone-capable carrier, which can be used to transmit the scheduling information (and other control information) for the extension carrier.
Thus, when a first base station is operating a component carrier as an extension carrier, another base station may operate a component carrier of the same frequency to transmit a control channel, a CRS and other such information more reliably, in the same general geographic area as the first base station, without significant interference because there is no corresponding control channel, CRS and other such information on the extension carrier operated by the first base station.
However, in communication systems in which extension carriers are employed, the cross-carrier scheduling from the stand-alone (legacy) component carrier can cause an increase in control channel (PDCCH) blocking and control channel (PDCCH) capacity can become a limiting factor of system performance. This is because of the additional control channel signalling required to schedule resources on multiple component carriers.